This invention relates to a master cylinder for hydraulic systems, for example, braking systems and, more particularly, to the provision of recuperation in such systems. British Patent Specification No. 1,333,050 relates to a hydraulic booster comprising a pedal actuated master cylinder, a power valve and a boost cylinder, and a later Patent Specification, No. 1,468,014, describes a version of the booster adapted to anti-lock operation by the addition of solenoid valves and another boost cylinder.
In such systems, recuperation is required in the pedal released condition to allow the braking (or clutch) system to self-bleed, i.e., any air bubbles that may be present in the fluid are allowed to travel upward to the reservoir. Recuperation may also be necessary to prevent a vacuum being induced in the braking circuit by the master cylinder on its return stroke in the event of a slight fluid loss from the brake circuit or when bleeding the brake circuit.
Recuperation may also be required in the event of expansion of the fluid in the system to ensure full release of pressure in the pedal released condition. It has been found that anti-lock operation introduces new conditions of operation that can result in problems using conventional recuperation means. Various methods of recuperation are known:
1. The use of seals passing over holes or slots. With this type of recuperation in anti-lock use, it is possible to have the seal cycling rapidly across the hole, resulting in premature wear and leakage of the seal.
2. Center valves may be used in a horizontally disposed cylinder. However, the possibility of air being trapped above the level of the valve can lead to incomplete self-bleeding. This may not be a problem in normal operation, but can aversely affect anti-lock operation.
3. A tilt valve may be used to overcome the difficulties referred to above, but due to its method of operation and the bulk thereof, it is not always a convenient device to use, in particular with the additional complication of an anti-lock unit.
Another proposal enabling recuperation is to arrange the piston such t hat it can be withdrawn from the cylinder bore to the extent of providing a recuperation flowpath past a seal between the piston and the bore. A serious deficiency of such an arrangement, however, is that the seal, usually a rubber seal mounted on the piston, tends to be extruded into the gap which remains between the piston and an enlarged portion of the bore when pressure has built up after re-entry of the seal lip into the bore at the beginning of the forward stroke.